ABSTRACT Across the life course, alcohol use reaches its peak for most individuals during young adulthood, and use at this time is associated with many acute negative consequences and potential longer-term effects. During this period, changes in alcohol use and heavy drinking have been found to be a function of social role status and contexts. Young adults, on average, tend to mature out of heavy drinking as they enter their mid/late-20's and take on new roles and responsibilities. However, the high prevalence of problematic drinking among those in their late teens and early/mid-20s, including being at highest lifetime risk for having an alcohol or substance use disorder, suggests that as a field we need to better understand not only major life changes (e.g., marriage, parenting), but also the many smaller life changes that occur during young adulthood. Very little is known about the micro-transitions of young adulthood, or the incremental steps/transitions (e.g., dropping out of school, starting/ending jobs, starting/ending relationships, relationships increasing in seriousness, moving in/out with parents, friends, romantic partners) that comprise major role transitions, including the perceived characteristics of these roles and transitions, and the impact on immediate and long-term changes in alcohol use and consequences. We need to develop and test models of risk that can lead to specific intervention strategies for hazardous alcohol use and consequences. The proposed research focuses on alcohol use during young adulthood and investigates empirical evidence for two developmental models that have been posited to explain why alcohol use increases during young adulthood: the Transitions Catalyst Model and the Transitions Overload Model. The long-term objectives of the proposed project are to extend prior research by taking a finer-grained approach with monthly assessments of young adults (N = 800, 18-25 years old, recruited via social networking) across two years and bi-annual surveys in Year 3 (total of 26 data points) to understand: 1) the multitude of social role micro-transitions and characteristics of these transitions both within and between domains and the impact on alcohol use and consequences, 2) whether changes in alcohol use precede or follow micro-transitions, and whether this is a function of changing drinking motives, and 3) how distinct profiles of role transitions correspond with longer-term problematic alcohol and other substance use in YA, including risk for abuse and dependence. The proposed application fits within NIAAA's strategic plan and has significant implications for public health, including identifying when to intervene (e.g., before or after certain transitions/life changes), what mechanisms to target (e.g., drinking motives, stress), and who to target.